Many different requirements are expected of the network layer in all-IP access networks (e.g. 4G cellular networks). Two in particular are mobility and QoS. The former enables users to communicate seamlessly with remote network nodes via the Internet wherever they are, whereas the latter enables users to receive different levels of service for certain types of traffic.
Mobility at the network layer is concerned with maintaining the routability of packet data to and from a mobile node when that mobile node moves away from its home access network. The main candidate for provision of this functionality is Mobile IP (MIP). Very briefly MIP relies on a Home Agent in the home access network to tunnel IP packets to the domain where the mobile node is attached. The mobile node forms a Care-of Address (CoA) that is globally topologically correct in the network to which it is attached. The Home Agent encapsulates packets that it receives addressed to the mobile node's home address in another IP packet addressed to the CoA. In this way packet data may still reach the mobile node even when it is away from the home network. Further details of Mobile IP can be found in RFC 3344, 3775 and 3776 to which reference is specifically made.
However, when a mobile node hands over to a new access router, binding updates are triggered to the Home Agent, etc. These binding updates can introduce unwanted delays and loss of packets, and thereby degradation in performance from the user's perspective. When attached to a particular wireless access network (such as a cellular network), a mobile node may change its point of attachment (i.e. access router) quite frequently (e.g. every few minutes or more often, particularly if on the move). Each change triggers configuration of a new CoA, followed by the necessary binding updates. Doing this frequently (e.g. every few minutes) is not practical.
Hierarchical Mobile IPv6 (HMIPv6) has been proposed (see RFC 4140) to address this problem. HMIPv6 provides a mobility agent known as a Mobility Anchor Point (MAP) in the access network. A MAP is a logical entity that handles micro-mobility for the mobile node. Micro-mobility is a change in point of attachment of the mobile node from one access router to another, both of which are within the same domain of the access network. Whenever this happens, the mobile node sends a binding update to the MAP (comprising a new Link local CoA or LCoA), but the mobile node's primary CoA (or Regional CoA or RCoA) remains unchanged. In this way the mobile node can move between access routers in the same administrative domain without having to send a binding update to the Home Agent. In contrast when the mobile node changes point of attachment to an access router in a different access network, this is a macro-mobility event i.e. requiring a binding update to be sent to the Home Agent of the mobile node.
It is possible that the mobile node may take the form of a moving network, a specific example of which is called a ‘network in motion’ or NEMO network. Network mobility support is concerned with managing the mobility of an entire network, viewed as a single unit, which changes its point of attachment to a fixed network infrastructure and thus its reachability in the network topology, most frequently the Internet. The mobile part of the network is referred to as a ‘moving network’, that can be installed in a train for example, and which includes one or more ‘mobile routers’ (MRs) which act as gateways to the moving network and connect it to the global Internet. Nodes behind the MR(s) can be classified as “Local Fixed Nodes” (LFNs) such as wired Internet access point on the train, Local Mobile Nodes (LMNs) such as mobile PDAs carried by personnel working on the train, and Visiting Mobile Nodes (VMNs) such as notebook computers, PDAs, portable media players, hand-held gaming devices and mobile telephones carried by passengers on the train. In most cases, the internal structure of the moving network will in effect be relatively stable (no dynamic change of the topology), subject to joining and leaving of VMNs and LMNs. The MR provides wireless access for the network nodes via access routers that are part of the fixed network infrastructure. Access routers include satellites, UMTS Node Bs, GSM base stations, DVB transmitters and wireless access points.
The mobility of moving networks does not cause the network nodes to change their own physical point of attachment, although they happen to change their topological location with respect to the global Internet. If network mobility is not explicitly supported by some mechanism, the mobility of the MR results in mobile nodes losing Internet access and breaking ongoing connections with correspondent nodes (hereinafter CNs) in the global Internet. In addition, the communication path between the network nodes and the CNs becomes sub-optimal, and nested mobility will cause yet worse routing paths.
The current solution to provide network mobility is promoted by the NEtworks in MOtion (NEMO) working group of the Internet Engineering Task Force (see www.ietf.org/html.charters/nemo-charter.html). “NEMO Basic Support” (see RFC 3963 at the same website) aims to provide connection continuity for nodes in the moving network.
NEMO Basic Support provides connection (or session) continuity (e.g. continuity for TCP connections) by creating a “bi-directional tunnel” between the MR and its home network. This bi-directional tunnel is formed by encapsulating IP packets to and from the network nodes in IP packets addressed between the MR and a Home Agent on the MR's home network. In this way traffic flows are routed via the MR and its Home Agent and the mobility of the moving network is transparent to all network nodes attached thereto.
We have realized that mobile routers that arrive within the signal coverage of an access network that operates both a micro-mobility protocol, such as HMIPv6, and a QoS protocol, face particular difficulties. In particular, when an access network operates both a mobility protocol (such as HMIPv6) and a QoS protocol (such as DiffServ or IntServ), the requirement that all packets to and from the moving network pass through a single mobility agent creates a bottleneck which will lead to congestion in the router hosting the mobility agent and at other routers nearby.
Furthermore, the utilization of the mobility agent's resources is always likely to be relatively high since all traffic within its domain must pass through it. This bottleneck can be particularly problematic for moving networks utilising aggregated QoS reservations, in which a high number of mobile terminals (e.g. ten, twenty, one hundred or more) may require the simultaneous and substantially seamless handover of ongoing sessions to a single mobility agent. We have realised that the main problem is the possibility that no single mobility agent within the new network is likely to have the capacity to support the resource requirements of the moving network as a whole. If so, the mobility agent will reject the resource request of the moving network, causing the mobile router either to blindly seek alternative mobility agents with which to make a resource reservation, or negotiate a lower resource reservation with the mobility agent, with the intention of either dropping the sessions of some mobile nodes, or proportionally reducing the QoS given to all sessions.
In both of these situations, handover latency will be significantly increased as the mobile router attempts to establish a reservation with a mobility agent that can accommodate the resource requirements of the moving network, assuming that there is another mobility agent available in that access network.